As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology techniques, operate by illuminating a sample with a probe beam, then detecting and analyzing the reflected radiation. Ellipsometry and reflectometry are two examples of commonly used optical metrology techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Changes in intensity are analyzed for reflectometry, while scatterometry is used when the structural geometry of a sample creates diffraction (optical scattering) of the probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur.
Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (such as is described in U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). Each of the above-listed patents is hereby incorporated herein by reference. Scatterometry generally refers to optical response information in the form of diffraction orders produced by periodic structures (e.g., gratings on a wafer). In addition, any of these measurement technologies, e.g., single-wavelength laser BPR or BPE, can be employed to obtain critical dimension (CD) measurements on non-periodic structures, such as isolated lines or isolated vias and mesas.
FIG. 1 shows a diagram for a typical optical metrology tool, in which an illumination source 102 creates a monochromatic or polychromatic probe beam. The probe beam is projected by one or more lenses 104, 106 onto the surface of a sample 108. The sample reflects at least a portion of the probe beam, and the reflected portion is transported to a detector 110. The detector transforms the received energy into corresponding output signals. A processor 112 analyzes the signals to measure the structure or composition of the sample.
A polychromatic probe beam typically is used for broadband measurements. The spectral range of these probe beams can be quite large, such as from near-infrared to deep ultra-violet. Creating broadband probe beams is technically challenging. Typically, the outputs of two or more different sources must be combined to create the desired spectral range. The combined outputs then must be filtered to remove undesired components, such as infrared components, as well as to maximize constancy and minimize light contamination over the desired spectral range.
Optical filters exist in a broad array of configurations. There are filters that absorb or reflect various wavelength bands. These filters can be composed of bulk materials like colored glasses, or composed of dielectric stacks of thin films, for example, that combine to make a filter with the desired properties. It is extremely difficult, if not impossible, to manufacture filters that have specific desired attenuation properties from the deep ultra violet to the infrared because of material properties of bulk materials, such as glasses, and materials used for thin films. For example, there are sources that operate from the deep UV (e.g., 190-nm) to the near-infrared (e.g., 900-nm). Filters for these sources must have high transmissivity in that range and simultaneously block all longer wavelength light (e.g., above 950-nm) in order to prevent overheating of the optical system. While standard approaches for this problem exist, these approaches tend to be highly inefficient. In addition, when an optical spectrum is desired that has a smooth intensity, as a function of wavelength, and the spectral distribution contains a number of intensity spikes, such as in the case of Xenon arc lamps, it is virtually impossible to clean the spectrum with conventional filters without severely impacting the overall light throughput.